Polyesters are known to possess good chemical and physical stability, high mechanical strength, durability, heat and chemical resistance and electrical insulation properties; and, therefore, have been widely used in manufacturing various industrial products. In particular, polyethylene terephthalate films, due to their good elasticity, dimensional stability and slipperiness, have been used as magnetic recording media, condensers, photographic films, industrial products, packaging and labelling materials and the like.
Recently, polyester films have been increasingly used as a paper substitute in a variety of applications. However, such polyester films differ from paper in clarity, color and rigidity; and are rather inconvenient for use due to their high density.
Extensive attempts have been made to develop low-weighted polyester films with a paper-like softness while maintaining their desired properties. For example, Japanese Patent Laid-open Publication Nos. 87-243120 and 90-206622 describe the incorporation of inorganic particles into a polyester; Japanese Patent Laid-open Publication No. 83-50625 discloses a method for preparing a low-weighted polyester film by incorporating a foaming agent in a polyester; and Japanese Patent Laid-open Publication Nos. 82-49648 and 88-168441 offer a method for preparing a low-weighted polymeric film with improved surface properties by blending a polyolefin resin with a polyester and extending the resultant mixture to form microvoids on the surface and inside of the film obtained therefrom.
However, such attempts have their own drawbacks and problems. When a large amount of inorganic particles are incorporated in a polyester film, its density tends to increase. In case a foaming agent is incorporated in a polyester, the microvoids formed become poorly dispersed and physical properties of the film are difficult to control. Further, in case a polyolefin resin is blended with a polyester, due to the poor heat resistance of the polyolefin, the mechanical properties of the resulting polymer film are apt to deteriorate. Also, since a polyolefin tends to generate and accumulate static electricity, the polyester film blended with a polyolefin may become highly electrostatic and suffer from low printability, which would limit its usage as a paper substitute.